This invention is related to light guides, or pipes, used for illumination. More particularly, this invention is directed to an illuminating light pipe that is used to illuminate an elongated area along which the light pipe extends.
Light pipes or light guides may be employed for illumination as well as for high speed signal communication. When used for illumination, light pipes may be used to transmit light from a source to a remote area to be illuminated, and may be used to illuminate an area along which the light pipe extends. If a light pipe has a smooth outer surface with few imperfections, the index of refraction between the light pipe and air will be sufficient to reflect most of the light, propagated through the light pipe or fiber, so most of the light will be transmitted between a light source at one end and the other end of the fiber optic cable or pipe. However, to illuminate the elongated area along the light pipe or fiber, light must be emitted laterally, and to uniformly illuminate this elongated strip or path, light should be emitted at comparable levels along the light pipe. One approach is to use a multi-fiber cable with fibers of different lengths arrayed to emit light from the ends of the individual fibers at points spaced along the length of the multi-fiber cable.
Optical fiber lighting apparatus may employ several techniques for scattering light propagating within the light fiber of a light pipe into lateral modes of illumination to illuminate the area surrounding the light pipe. One such approach is to employ scattering centers within the core to scatter propagating light outwards, so that light is emitted laterally in the vicinity of the scattering centers. Another approach is to roughen portions of the core layer at the interface with the clad layer of the optical fiber, to scatter light impinging on the core-clad interface. By roughing the surface, the index of refraction is altered, so that more light can be laterally emitted from the light pipe or fiber. Although the core dimensions of the fiber or pipe remain constant for these two approaches, the light pipe is not uniform along the length of the pipe. Either discrete areas along the axis or length of the fiber or pipe must be roughened, or the scattering centers must be added at discrete locations. Therefore, the pipe or fiber typically cannot be fabricated on a continuous basis, or if a continuous fiber is used, it must be subjected to secondary operations to roughen the surface.
If the surface of a cable is roughened along the entire length of the fiber, either at the core-clad interface or along the exterior of a fiber that does not include a clad layer, light will sometimes not be emitted uniformly along the length of the fiber or pipe. A large portion of lateral light emission might occur adjacent to the light source and the lateral light emission might be significantly less near the remote end of the light pipe. A light pipe that is continuously roughened in this manner is not preferred for use in providing substantially uniform illumination along an elongate surface or area, such as along a step or the edge of a structure.
Another approach that has been employed is to position reflectors along one side of a light pipe or fiber optic cable so that light emitted from one side will be reflected back onto an area on the opposite side of the light pipe, such as employing multiple reflectors with progressively varying indices of reflection located along the length of the cable. This approach relies on the variation of the index of reflection of the separate reflectors to overcome the variation in the illumination along the length of the cable. One other approach varies the geometry of the cable along its length.
It is desired to provide uniform illumination along a surface or edge of a structure to highlight the structure for safety or other reasons. For example, it is often desirable to use a light pipe to illuminate a step up surface on a motor vehicle to increase the visibility and therefore the safety of such a feature. For both practical and aesthetic reasons, the illumination should be uniform along the length of such an elongate structure, or at least the feature should be comparably illuminated along its length. Since the light pipe is intended to illuminate the feature, a slight light intensity variation that would not be noticeable to an observer or user would still be acceptable. One attempt to provide a low cost light pipe of this type was to paint one side of a light pipe that would be mounted in a slot or channel on the vehicle. The paint would provide an opaque surface on one side that would hopefully reflect light back through the light pipe onto the exterior of the structure on which this light pipe was mounted.
The present invention provides a light pipe that may have a constant cross sectional area from which light may be laterally emitted at substantially uniform levels over the length of the light pipe. This light pipe may therefore be used in a number of cost sensitive applications, such illuminating an elongate surface to provide greater safety for users.
Embodiments of the invention include a thermoplastic light transmitting strip or guide for illuminating an elongated area between a location relatively proximate to a light source and a location relatively remote from the light source. The light intensity at the relatively remote location is approximately comparable to the light intensity at the relatively proximate location. The light pipe may include a substantially translucent and relatively clear member that may have a length equal to the distance between the relatively proximate location and the relatively remote location. The light guide, or pipe, may also include a substantially opaque reflective member extending adjacent the light guide proximal to the translucent member. A space or air gap may be formed between the opaque reflective member and the translucent member. A band of reflective material may be located proximal the translucent member on an outside surface and adjacent the air gap.
Embodiments of the invention also provide methods of fabricating a light pipe by co-extruding the translucent member and the opaque reflective member while simultaneously, or near simultaneously applying the band of reflective material to the exterior surface of the translucent member during its extrusion. The reflective member may be fabricated from the same resin used to fabricate the clear light guide with fillers added to make the reflective member relatively opaque. The band of reflective material may also be the same material as used in the reflective member. The band of reflective material may also be supplied from the same extrusion feed as the reflective member.